Tunguska is a computer emulator and virtual
machine that uses numerical base three (ternary)
number system for its lowest-level operations
instead of the commonly used base two (binary)
number system. Its goal is to provide a useful
experimental platform, on par with early personal
computers.

Slirp is a (C)SLIP/PPP emulator based on the
original BSD sources. It allows the user to pull
up a full internet connection over a shell
connection, and features sharing multiple modems
for additional bandwidth.

VisualOS is an educational visual simulator of an operating system for GNOME/GTK+. It represents a working operating system visually, allowing the user to select the different algorithms to use for each of the simulated subsystems: CPU, Memory and disk I/O.

PalmOrb is software that allows you to use your PalmOS device as an LCD status display for your computer. You can display system stats (such as CPU load graphs, CPU temperatures, free disk space), news alerts, stock indexes, WinAmp graphs, etc. It works by pretending to be (or emulating) a real LCD (a Matrix Orbital LK204-25). You can use existing software that expects a Matrix Orbital display.

AtariSIO is a Linux kernel driver module plus a
small set of tools that implement the Atari 8-bit
(400/600/800/XL/XE) computer series SIO protocol.
The kernel driver handles the low-level parts and
provides a simple interface for user space
applications. This package contains two tools:
atariserver, an Atari disk drive emulator (similar
to SIO2PC or APE for MSDOS), and atarixfer, a tool
to transfer disk images to/from a connected Atari
disk drive.

J80 is a Z80 emulator with a standard BIOS for a
complete Microcomputer with working CP/M 2.2, CP/M
3.0, and one simple ZX Spectrum 48/128K emulator.
The spectrum 128K version emulation is incomplete
but working. The goal of this emulator is that the
'hardware' is built on-the-fly, reading the
configuration from one file. This makes it very
easy to add new peripherals or features.

Yet Another Machine Simulator (YAMS) is a simple
yet realistic machine simulator, which is mainly
aimed for teaching purposes on operating systems
courses. It simulates one or more MIPS32 CPUs and
I/O devices such as disks, terminals, and network
cards. Additional devices can be implemented as
separate programs that use its I/O plugin
interface. The device properties are very
configurable. High performance was not a priority
factor in its implementation since the main
purpose is educational, so YAMS is not an answer
for people looking for a fast MIPS
emulator/simulator.

Romeo is a Java-based MAME frontend. It supports
XMame and the Windows command line, and it has a
lot of features. This application is skinnable.
It can automatically download ROMs, screenshots,
flyers, and other related types of media. A
different config file can be used for each game.